The invention pertains to a method for operating a system for a thermodynamic cycle, to a control unit for a system for a thermodynamic cycle, to a system for a thermodynamic cycle, and to an arrangement consisting of an internal combustion engine and a corresponding system.
Systems of the type in question here and methods for operating them are known. A system of this type usually comprises a circuit, through which a working medium is conveyed by a feed pump. This medium is vaporized in a evaporator and sent to an expansion device, in which it is expanded. Some of the heat absorbed by the working medium in the evaporator in converted to mechanical work. After the expansion, the working medium is cooled, in particular condensed, in a condenser, after which it is sent back to the feed pump again. A typical cycle for a system like this is the Clausius-Rankine cycle. A modification of this is the organic Rankine cycle, in which an organic working medium is typically used, which can be vaporized at a lower temperature level than water. Thus the organic Rankine cycle is especially suitable for making use of waste heat in industry, for using the waste heat of internal combustion engines, or for use in geothermal power generating plants, for example. Systems are known in which the evaporator has multiple flow channels. This can serve the purpose, for example, of making it possible to include several heat sources into the cycle; in addition, a multi-flow configuration of a single, integral evaporator can be advantageous for manufacturing reasons. When several evaporator flow channels of this type are operated in parallel, however, there is the problem of increased susceptibility to thermodynamic instabilities. In particular, the so-called Ledinegg instability can occur: When vaporization begins prematurely in one of the evaporator flow channels, the pressure drop in this channel increases sharply. This results in turn in a sharp decrease in the flow of medium through this evaporator flow channel as a result of the pressure relationships, and this causes the effect to become even more pronounced. The heat transfer in the evaporator is sharply reduced, because the evaporator flow channel in question is almost completely blocked. Thus the efficiency and the power output of the system decrease. There is also the danger that the working medium in the blocked flow channel can become unallowably superheated. In this case, deposits can also form, which permanently reduce the heat transfer in the evaporator and thus reduce the energy yield of the overall system over the long term. When working medium suddenly starts to flow through the blocked evaporator flow channel again, thermal shock can occur, leading to irreversible damage at least to the evaporator flow channel in question, if not to the entire evaporator.